1. Field of the Invention
The invention relates to a grinding/polishing cover sheet for placing on a rotatable grinding/polishing disc, particularly for the treatment of materialographic samples.
2. The Prior Art
The treatment of materialographic samples in a grinding/polishing machine is usually performed in three steps, viz. a plane grinding step, a fine grinding step and a polishing step, but according to circumstances one or more intermediary steps may be interposed.
The cover sheets employed today are usually not in themselves used for grinding, but serve as a substrate for a grinding/polishing agent, which is sprayed or otherwise applied to the surface of the cover sheet, either automatically or manually. During the grinding a lubricant may also be applied. The grinding grains employed may, e.g., be diamonds, aluminum oxide or silicon carbide of different fineness for the different steps.
Grinding/polishing cover sheets are known which have a surface layer consisting of a cloth of textile fabric or woven or non-woven synthetic fibres, cf. FR 2226068. The cloth serves as a substrate for a grinding/polishing agent and may on its rear side have an adhesive layer with a detachable protective layer, so that it can be glued directly to the surface of a grinding/polishing disc, or the cover sheet may comprise additional layers including e.g. a supporting foil with an adhesive layer.
Cover sheets of the kind described have the advantage that they are cheap to manufacture and easy to handle, so that they may be used over and over again, until they have been worn out, and can thereafter be discarded without substantial loss. They have, however, the disadvantage that they have a soft surface so that the samples can press themselves into the surface layer, whereby the edges of the samples are rounded, and a relief pattern is formed between different phases of different hardness in the surface, which is in many cases not acceptable.
It is also known to use hard coatings consisting of a composite material, e.g., of epoxy with particles of one or more metals or ceramic materials embedded therein. In the prior art, cf., e.g., DK 143096, such coatings are moulded on or glued to a self-supporting rigid plate, e.g., in the form of mutually separated segments, whereafter the surface of the coating is straightened by turning and/or grinding. In this manner a tool is formed, which can be mounted as grinding/polishing disc in a grinding/polishing machine.
In the use of such a tool the hard surface of the coating material is non-uniformly worn, the wear being greatest adjacent the center and decreasing towards the periphery. Thereby the surface of the coating will gradually assume a concave shape, which results in obliquity of the surfaces of the samples. When the obliquity reaches a certain maximum permissible limit value, the surface of the coating can be straightened anew by a turning and/or grinding operation, and this can be repeated several times, but each time it is a cumbersome and time consuming operation which interferes disturbingly with the progress of the work and thereby impairs productivity. When renewed straightening of the coating is no longer possible, the tool must be discarded in its entirety, whereby a substantial initial expense is lost.
It is the object of the invention to provide a grinding/polishing cover sheet by which the drawbacks of the previously known systems are avoided.